Bug 1511375 - Update the visual viewport size in UpdateResolution() if either the zoom or the display size might have changed. r=kats

Differential Revision: https://phabricator.services.mozilla.com/D13578

--HG--
extra : moz-landing-system : lando

layout/base/MobileViewportManager.cpp

@@ -329,15 +329,19 @@ void MobileViewportManager::UpdateResolution(
     }
   }
 
-  // If the zoom has changed, update the pres shell resolution and the
+  // If the zoom has changed, update the pres shell resolution accordingly.
-  // visual viewport size accordingly.
   if (newZoom) {
     LayoutDeviceToLayerScale resolution = ZoomToResolution(*newZoom, cssToDev);
     MVM_LOG("%p: setting resolution %f\n", this, resolution.scale);
     mPresShell->SetResolutionAndScaleTo(resolution.scale);
 
     MVM_LOG("%p: New zoom is %f\n", this, newZoom->scale);
-    UpdateVisualViewportSize(aDisplaySize, *newZoom);
+  }
+
+  // The visual viewport size depends on both the zoom and the display size,
+  // and needs to be updated if either might have changed.
+  if (newZoom || aType == UpdateType::ViewportSize) {
+    UpdateVisualViewportSize(aDisplaySize, newZoom ? *newZoom : zoom);
   }
 }